In order to remove particulates, organic matter, metals, etc. from the surfaces of electronic materials such as silicon substrates for semiconductors, glass substrates for liquid crystals, quartz substrates for photomasks, etc., functional cleaning water prepared by dissolving a specific gas in ultrapure water by a gas dissolving device, and then adding trace amounts of chemicals as necessary has come to be used in place of high concentration chemical solutions. As the specific gas used in the functional cleaning water, in addition to carbon dioxide gas, hydrogen gas, oxygen gas, ozone gas, there are also inert gases, etc. such as noble gases. Above all, methods for dissolving carbon dioxide gas and hydrogen gas are being considered.
In particular, carbonated water is being used in order to rinse a cleaned object surface without electrically charging the cleaned object or nozzle. In other words, when using ultrapure water in cleaning, due to the magnitude of the insulation property thereof, the cleaned object may be charged due to the friction with the cleaner. If the cleaned object electrically charges, for example, such as in a case of there being a fine circuit pattern on the cleaned object, the circuit thereof may be damaged. Carbonated water with enhanced conductivity by dissolving carbon dioxide gas in ultrapure water is being used in order to prevent this.
The carbon dioxide gas concentration of this carbonated water is set to a low concentration at 1 to 100 mg/L, due to only being for imparting conductivity to ultrapure water. In order to efficiently dissolve the above-mentioned gases, a gas dissolving membrane module equipped with a gas-permeable membrane having a property of allowing only gas to permeate is being practically used (for example, Patent Document 1). When using the gas dissolving membrane module, gas dissolved water free of bubbles can be produced easily.
In addition, a method of using a porous hollow fiber membrane as the hollow fiber membrane has been proposed (Patent Document 2).
In the case of using a porous hollow fiber membrane in the gas dissolving membrane module, the membrane is hydrophilized from use over a long time period, and water will leak to the gas side and block the membrane surface, and thus there is concern over the initial carbon dioxide gas addition ability no longer being able to be obtained.
As a non-porous gas separation membrane, an ethylene-vinyl alcohol-based polymer composite membrane for a gas separation membrane consisting of an ethylene-vinyl alcohol-based polymer and an amine compound (Patent Document 3); a composite hollow fiber membrane having a three-layer structure in which a homogeneous layer consisting of a linear polyethylene is sandwiched by two porous layers (Patent Document 4); etc. have been presented.
In the case of using a non-porous hollow fiber membrane, since a non-porous membrane is present between the gas side and liquid side, water will not leak to the gas side even if using for a long time period; however, due to the water vapor permeability of the membrane, the water vapor having permeated condenses on the gas side from the change in water temperature, fluctuation in pressure, etc., and thus there has been concern over the condensed water (drain) blocking the membrane surface nonetheless.
If the condensed water is a small amount, the influence exerted on the performance of the gas dissolving module will be slight; however, when the amount of condensed water increases, it gradually accumulates upwards from the bottom of the gas chamber, the effective area of the gas permeable membrane contributing to the dissolution of gas decreasing, whereby the performance of the gas dissolving module declines, and the gas concentration contained in the functional cleaning water decreases.
The gas dissolving module feeds gas to be dissolved into the gas chamber, causing the gas component to migrate through a membrane to a water chamber, and the migration of water vapor in the opposite direction through the membrane is almost never taken into account, and thus countermeasures for condensed water have not been considered sufficiently.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2000-271549
Patent Document 2: Japanese Unexamined Patent Application, Publication No. H02-279158
Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2010-155207
Patent Document 4: Japanese Unexamined Patent Application, Publication No. H11-47565